Polysulfide derivatives

ABSTRACT

Polysulfide derivatives, having the general formula (A-S-S-)2 X-S-S-A processes for their production and their use for crosslinking of reversion-stable rubber vulcanizates are provided. Crosslinking with reduced reversion and loss of elastomeric properties with age is achieved by the crosslinking system provided by the invention.

This is a division of application Ser. No. 07/624,009 filed Dec. 7,1990, now U.S. Pat. No. 5,241,108.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to concurrently filed copending applicationSer. No. 07/624,176 , entitled "Aromatic Dithiocarboxylic AcidVulcanization Accelerators and Method of Their Use" by Horpel et al.,based on German Application P 39 41 001.3, filed Dec. 12, 1989.

BACKGROUND OF THE INVENTION

The invention relates to new sulfide derivatives of aromaticdithiocarboxylic acids, a process for their production and their use asnitrosamine-free crosslinking agents in rubbers for improvements inreversion stability. The polysulfide derivatives according to theinvention have the general formula

    (A--S--S--).sub.z X--S--S--A'

Crosslinking rubber with sulfur, or sulfur donors and acceleratorsystems, generally yields vulcanizates, whose polymer chains in thebeginning phase are linked by polysulfide sulfur crosslinking.Polysulfide-linked vulcanizates with a great number of crosslinks leadto optimal results (cf. A. V. Chapman, M. Porter "Sulfur VulcanizationChemistry" in "Natural Rubber Science and Technology," A. D. Roberts, edOxford Press 1988). This applies to mechanical strength and especiallyto tear propagation resistance and also to abrasion resistance. Butthere is a disadvantage in such sulfur-containing systems insofar asthey are subject to reversion which encompasses the decline of thevulcanizate properties during the vulcanization process and theanaerobic aging of the resultant vulcanized products during dynamicstress. The theories on the mechanism are that under thermal stress abreakdown of the polysulfide bridges to the monosulfide arrangementoccurs. The sulfur thus being released is added to polymer segments andthus increases their glass transition temperature (Tg) but alsocatalyzes the attack of the oxygen on the vulcanizate. Both effects leadto damage of the crosslinks. These secondary reactions of the sulfur,which already begin during the vulcanization process as a function ofthe reaction time and reaction temperature, substantially affect theperformance limits of the elastomers and therefore require limitation inview of the optimal configuration of the production process (see in thisconnection also A. V. Chapman, M. Porter in the above-cited article).

The use of the so-called EV systems represents an attempt to reduce thesecondary reactions of the sulfur; they are system which consist ofincreased amounts of vulcanization accelerators with simultaneouslyminimal amounts of sulfur. But as a result of the fasterprevulcanization, the processing reliability is adversely changed. Thereis added to this--and this is even more important--the preprogrammedimpairing of the abrasion resistance, the tear propagation resistanceand the cord adhesion, as is consistent with known tire technology (cf.P. M. Lewis "Vulcanisate structure and it effects on properties" inNR-Technologie, Quarterly Volume 17, Part 4, page 57ff., 1986).

To provide thermally stable vulcanizates, polysulfide derivatives wereproposed (cf. DE-AS 22 65 382), which with the addition of thermalenergy, release polysulfide segments. These polysulfide derivativescontain conventional accelerator systems, e.g., dithiocarbamoylstructures, as initial groups (A). But a considerable disadvantage ofthe dithiocarbamate system is that during the vulcanization process,nitrosamines are formed, which result by the nitrosation of thedecomposition products (amines) of the dithiocarbamates unstable underthese conditions. According to today's knowledge, nitrosamines areconsidered a health hazard, since they form mostly carcinogenicmetabolites (cf. Umschau 1985 (1), 24). Amine-free polysulfidederivatives as crosslinking agents are also known, but they exhibit acompletely unsatisfactory level of performance.

SUMMARY OF THE INVENTION

The principal object of this invention, therefore, is to providethermally stable vulcanizates while avoiding the formation ofnitrosamines.

To attain the objects of this invention, this invention provides asulfide derivative of an aromatic dithiocarboxylic acid of the formula

    (A--S--S--).sub.z X--S--S--A',

wherein

A and A' are, independently, amine-free aromatic thiocarbonyl groups offormula I ##STR1## wherein n, m, p, q and r are 0 or 1 and the sum ofn+m+p is at least 1,

R¹ and R² are, independently, C₁ -C₁₀ -alkyl radicals, alkenyl radicalswith up to 10 carbon atoms (C₁ -C₁₀) or aralkyl radicals with up to 15carbon atoms,

z is a natural number with 0<z<50, and

is a divalent (z=1) or polyvalent (z>1) linear or branched aliphatic,cycloaliphatic or araliphatic compound, or polymer, preferably anethylenically unsaturated hydrocarbon polymer, having a degree ofpolymerization up to 50 and a valence of at least two, and where z=l, Xis a C₁ -C₃₀ -alkylene radical, or an alkenylene radical with up to 30carbon atoms (C₁ -C₃₀), or aralkylene radical with up to 40 carbon atoms

and where 1<z≦4, X is a polyvalent C₁ -C_(20z) alkane radical or apolyvalent alkene radical with up to 20z carbon atoms or a polyvalentarylalkane radical with up to 20z carbon atoms.

Also provided are methods for the production of said sulfide derivativesof an aromatic dithiocarboxylic acid, wherein a dithiocarboxylic acid ofthe formula ##STR2## in which n, m, p, q and r are 0 or 1 and the sum ofn+m q is at least 1, R¹ and R² are, independently, C₁ -C₁₀ -alkylradicals, alkenyl radicals with up to 10 carbon atoms (C₁ -C₁₀), oraralkyl radicals with up to 15 C atoms, is reacted at a temperaturebetween -10° C. to +60° C. and pH between 6 and 9 with the Bunte salt ofa divalent or polyvalent alkyl compound, of the formula

    (NaO.sub.3 SS).sub.z X--SSO.sub.3 Na,

in which

z represents a natural number between 0 and 50, and

X is a divalent (z=l) or polyvalent (z greater than 1) linear orbranched aliphatic, cycloaliphatic or araliphatic compound or polymerhaving a degree of polymerization of up to 50 and a valence of at leasttwo,

for z=l, X is: an alkylene radical with 1 to 30 carbon atoms, analkenylene radical with up to 30 carbon atoms (C₁ -C₃₀), or

an aralkylene radical with up to 40 carbon atoms for 1<z≦4, X is apolyvalent C₁ -C_(20z) -alkane, polyvalent alkene with up to 20z carbonatoms (C₁ -C_(20z)), or

polyvalent arylalkane with up to 20z carbon atoms.

In another aspect, this invention provides a process for vulcanizingrubber which comprises blending a compound of Formula I and a primaryaccelerator with an organic polymeric rubber to form a mixture andheating the mixture to a temperature in the range of 100° C. to 250° C.

Furthermore, crosslinked rubbers produced by this vulcanization processare provided.

An underlying inventive concept of the principal object of the inventionis manifested by employing instead of oligomer S bridges for crosslinks,stable carbon-containing bridges (X) with amine-free phenol thiocarbonylinitial groups are combined according to the above described formula:

    (A--S--S--).sub.z X--S--S--A'.

Preferred initial groups A and A' are4-hydroxy-3,5-dialkyl-benzene-thiocarbonyl groups, and optionallyinstead of the phenol group a phenolate group is also advantageous.Especially preferred is the4-hydroxy-3,5-di-tert-butyl-benzene-thiocarbonyl initial group or thecorresponding phenolate. These can be obtained from the aromaticdithiocarboxylic acids described above and produced in accordance withthe procedures set forth in the examples or the procedures referred toin copending application Ser. No. 07/624,176, filed concurrentlyherewith.

For crosslinking bridges X, the large number of possible compounds islimited by the availability of initial compounds of dichlorides anddithioles. Preferred crosslinking bridges X are divalent linear alkylenegroups, and especially preferably, z=1, X is ethylene and R¹ and R.sup.2 are lower alkyl.

Other crosslinking bridges include ##STR3##

Examples of substituents for R¹ and R² include ##STR4##

Natural and synthetic rubbers are suitable as rubbers for thecrosslinking process according to the invention. Preferred syntheticrubbers are, e.g., styrene-butadiene copolymers, polybutadienes,polyisoprenes, integral rubbers (as described in DE-OS 37 10 002, 37 24871, 38 04 574) or their blends, but also nitrile, chloroprene, butyl,EPDM rubbers and others.

There are no limitations in regard to other usual rubber additives suchas, e.g., fillers, plasticizers, tackifiers, accelerators, activators,stearic acid, waxes, aging and ozone protective agents, expandingagents, dyes as well as pigments. The production of a vulcanizing systemwith the above-mentioned rubber additives takes place according to theusual processes known to one skilled in the art.

The vulcanization is performed at temperatures between 100° and 250° C.,preferably between 120° and 240° C. The vulcanization also occursaccording to usual processes known to one skilled in the art.

The polysulfide derivatives according to the invention exhibitadvantages in the formulations to be used relative to reversionstability after prolonged vulcanization time and vulcanizationtemperatures. At the same time they exhibit a considerably smallerdecline in performance from elastomer aging and are safe in regard todanger of generating carcinogenic nitrosamines.

They combine in themselves a double function of crosslinking reagent(bis-thio-alkane bridges) and antioxidant (sterically hindered phenol),and the latter exhibits the additional advantage of being fixed in thenetwork.

The process for the production of the above-mentioned polysulfidederivatives by reaction of a dithiocarboxylic acid of the formula

    A--SH,

where A has the above-mentioned meaning, with the Bunte salt of thedivalent or polyvalent alkyl compound of formula

    (NaO.sub.3 SS--).sub.z X --SSO.sub.3 Na,

where and X and z also have the above-mentioned meanings, is preferablyconducted so that 2 moles of the dithiocarboxylic acid as a solution ina water-miscible organic solvent is added to 1 mole of the Bunte salt asan aqueous or organic water-miscible solution at temperatures of -10° to+60° C., preferably 20° to 40° C., and the pH of the reaction solutionand all educts are kept constant between 6 and 9, preferably between 7and 8.

The preferred organic solvent is tetrahydrofuran (THF).

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing and in the following examples, all temperatures are setforth uncorrected in degrees Celsius and unless otherwise indicated, allparts and percentages are by weight.

The entire disclosures of all applications, patents and publications, ifany, cited above and below, and of corresponding application FederalRepublic of Germany P 39 41 002.1, filed Dec. 12, 1989, are herebyincorporated by reference.

Production examples

1. Bunte salt of 1,2-dichloroethane

40.16 g of 1,2-dichloroethane (97.5%) is refluxed with 198.40 g ofsodium thiosulfate pentahydrate in a mixture of 800 ml of water and 650ml of ethanol for 15 hours. The resulting bis-Bunte salt is used withoutfurther work up for the next stages.

2. DBCA: 4-Hydroxy-3,5-di-tert-butyl-benzenedithiocarboxylic acid orDBCAK: the corresponding potassium salt

1031.7 g of di-tert-butyl phenol under nitrogen is dissolved in 2.5liters of THF in a nitrogen-flushed 6-liter three-neck flask with refluxcondenser, dropping funnel and stirrer. After introduction of 561.0 g ofKOH powder (exothermic reaction), the flask contents are cooled to 10°C. At this temperature, 380.5 g of carbon disulfide is added over 5hours and then allowed to post-react for 15 hours at that temperature.The resulting solid is suctioned off, rewashed with cold acetone anddried at room temperature in a vacuum drying cabinet. 1,525 g of thecrude product of DBCAK was present (85% yield).

3. DBEB:1,2-Bis-(4-hydroxy-3,5-di-tert-butyl-benzenethio-carbonyldithio)ethane(More proper name according to IUPAC:bis-[4-hydroxy-3,5-di-tert-butyl-benzene-dithiocarboxylic acid(dithioperoxo)]1,2-ethanediyl ester)

294.1 g of DBCAK (from 2.) as 10% aqueous solution is mixed with 1,200ml of THF and adjusted to pH 7 with acetic acid. Into this receiverthere are added, on the one hand, 1,516.8 g of Bunte salt solutionfrom 1. (corresponds to 0.4 mole of Bunte salt) and parallel with it,67.3 g of a 37% formaldehyde solution over a period of 3 hours. Then thepH is adjusted to 8 and allowed to post-react for 15 hours at roomtemperature as well as 5 hours at 40° C. 171 g of crude product can beisolated, which can be recrystallized from petroleum ether.

    ______________________________________                                        Elementary analysis:                                                                          C      H        O    S                                        ______________________________________                                        Cld:            58.67  7.08     4.88 29.36                                    Fnd:            58.37  7.15          27                                       ______________________________________                                    

Characteristic IR bands (in cm⁻¹):

3 600, 2 960, 1 580, 1 215

¹³ C NMR spectrum (in CDCl₁₃): Peaks at 30.015, 34.277, 36.611 , 76.570,76.922, 125.295, 135.564, 136.036, 159.086, 225.316 ppm.

Vulcanization examples Experimental

All the tread vulcanizates described below were produced as follows:

With an Internal Mixer, model GK 2 of the company Werner and Pfleiderer,at a rotor speed of 50 rpm and a jacket temperature of 40° C., thefollowing base mixture is prepared:

    ______________________________________                                        rubber         100                                                            ZnO             3 phr                                                                        (lbs. per hundred rubber)                                      stearic acid    2 phr                                                         aromatic plasticizer oil                                                                     10 phr                                                         Black N-339    50 phr                                                         crosslinking agent                                                                           see tables                                                     ______________________________________                                    

The modulus at 100 or 300% elongation was determined according to DIN 53504.

The tear resistance was determined according to Pohle (cf. S. Bostroem,Kautschuk-Handbuch [Rubber Manual], Volume 5, page 123).

The permanent set was determined according to DIN 53 518.

The hardness (Shore A) was determined according to DIN 53 505.

The rebound (elasticity) was determined according to DIN 53 512.

The abrasion was determined according to DIN 53 516.

The compression set was determined according to DIN 53 517.

The heat buildup test takes place according to DIN 53 533, part 3, withthe Goodrich Flexometer, identified in the test part as method 1.Moreover, the following more onerous conditions were selected, which areidentified in the test part as method 5:

Load: 500 N, starting temperature: 50° C., time: 25 minutes.

Ball fatigue test at the respective stresses was determined according toS. Bastroem, Kautschuk-Handbuch, 5th volume, Berliner Union, Stuttgart,pp. 149, 150.

The vulcametry was performed according to DIN 53 529.

                  TABLE I                                                         ______________________________________                                        Example 1                                                                     Vulcanization of a premasticated NR (Mooney 70;                               RRS No. 3) with DBEB (7.0 phr) at elevated                                    reaction temperature (1st part).                                                     Example                                                                       Vulcanization temperature                                                     150° C.                                                                      180° C.                                                          Vulcanization time in min.                                                    30    5       10      15    20    30                                   ______________________________________                                        Vulcameter 150° C.                                                     t.sub.10 min                                                                           5.7                                                                  t.sub.90 min                                                                           12.1                                                                 Delta t min                                                                            6.4                                                                  Tensile  21.6    20.8    19.1  18.9  18.0  17.2                               strength MPa                                                                  Elongation at                                                                          581     528     498   476   468   445                                break %                                                                       Modulus                                                                       100% MPa 1.4     1.5     1.6   1.6   1.6   1.5                                300% MPa 8.3     8.8     9.1   9.2   9.3   9.1                                Tear resis-                                                                            81      84      78    72    72    69                                 tance N/mm                                                                    Permanent                                                                              10      10      10    11    10    11                                 set %                                                                         Hardness                                                                      22° C. Sh.A                                                                     62      62      61    62    62    61                                 75° C. Sh.A                                                                     58      54      54    53    54    53                                 Elasticity                                                                    22° C. %                                                                        47      46      47    47    47    46                                 75° C. %                                                                        58      57      56    56    56    56                                 DIN abrasion                                                                           133     125     123   131   135   134                                mm.sup.3                                                                      specific 1.10    1.10    1.10  1.10  1.10  1.10                               weight                                                                        g/mm.sup.3                                                                    ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        Comparison example 1                                                          Vulcanization of a premasticated NR (Mooney 70;                               RRS No. 3) with the vulcanization system                                      sulfur/CBS (2.5/0.6 phr) at elevated reaction                                 temperature                                                                          Example                                                                       Vulcanization temperature                                                     150° C.                                                                      180° C.                                                          Vulcanization time in min.                                                    30    5       10      15    20    30                                   ______________________________________                                        Vulcameter 150° C.                                                     t.sub.10 min                                                                           7.6                                                                  t.sub.90 min                                                                           12.5                                                                 Delta t min                                                                            4.9                                                                  Tensile  23.3    18.8    17.5  14.5  13.8  11.9                               strength MPa                                                                  Elongation at                                                                          558     498     424   510   526   483                                break %                                                                       Modulus                                                                       100% MPa 2.0     1.6     1.3   1.3   1.1   1.1                                300% MPa 10.1    8.5     6.6   5.7   5.4   5.3                                Tear resis-                                                                            74      64      26    19    18    16                                 tance N/mm                                                                    Permanent                                                                              16      15      13    9     11    10                                 set %                                                                         Hardness                                                                      22° C. Sh.A                                                                     61      58      55    54    53    52                                 75° C. Sh.A                                                                     57      52      46    45    44    43                                 Elasticity                                                                    22° C. %                                                                        48      50      47    45    46    44                                 75° C. %                                                                        60      60      54    51    50    49                                 DIN abrasion                                                                           129     121     148   189   224   247                                mm.sup.3                                                                      specific 1.10    1.10    1.10  1.10  1.10  1.10                               weight                                                                        g/mm.sup.3                                                                    ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        Example 2                                                                     Vulcanization of an SBR 1 500 with DBEB                                                7 phr DBEB                                                                      Vulcani-                                                           Vulcanization                                                                            zation at                                                          system     150° C.                                                                          after 7 days aging at                                    Properties in 40 Min.                                                                              100° C.                                                                        110° C.                                                                      120° C.                                                                      130° C.                       ______________________________________                                        Tensile strength                                                                         20.9      16.2    15.9  13.2  5.8                                  MPa                                                                           Elongation at                                                                            478       278     247   181   58                                   break %                                                                       Modulus                                                                       100% MPa   2.2       3.6     4.1   5.1   --                                   300% MPa   10.6      --      --    --    --                                   Tear resistance                                                                          28        26      16    9     --                                   N/mm                                                                          Permanent set %                                                                          9         4       3     2     1                                    Hardness                                                                      22° C. Sh.A                                                                       64        68      71    73    76                                   75° C. Sh.A                                                                       60        63      68    69    73                                   Elasticity                                                                    22° C. %                                                                          45        52      50    53    54                                   75° C. %                                                                          60        62      63    62    66                                   DIN abrasion                                                                             95        120     129   138   158                                  mm.sup.3                                                                      specific weight                                                                          1.12      1.12    1.12  1.12  1.12                                 g/mm.sup.3                                                                    Compr. Set 24                                                                            17        13      12    13    12                                   h/70° C. %                                                             ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        Comparison Example 2                                                          Vulcanization of an SBR 1500 with sulfur/CBS                                           Sulfur/CBS = 2.2/1.2 phr                                                        Vulcani-                                                           Vulcanization                                                                            zation at                                                          system     150° C.                                                                          after 7 days aging at                                    Properties in 40 Min.                                                                              100° C.                                                                        110° C.                                                                      120° C.                                                                      130° C.                       ______________________________________                                        Tensile strength                                                                         22.0      18.3    9.5   6.2   3.6                                  MPa                                                                           Elongation at                                                                            427       309     124   35    --                                   break %                                                                       Modulus                                                                       100% MPa   2.4       3.5     7.1   --    --                                   300% MPa   13.1      17.7    --    --    --                                   Tear resistance                                                                          31        26      6.8   --    --                                   N/mm                                                                          Permanent set %                                                                          8         5       2     1     1                                    Hardness                                                                      22° C. Sh.A                                                                       66        73      78    82    87                                   75° C. Sh.A                                                                       60        69      75    78    85                                   Elasticity                                                                    22° C. %                                                                          44        60      57    41    38                                   75° C. %                                                                          68        65      63    56    53                                   DIN abrasion                                                                             108       118     159   167   220                                  mm.sup.3                                                                      specific weight                                                                          1.12      1.12    1.12  1.12  1.12                                 g/mm.sup.3                                                                    Compr. Set 24                                                                            14        11      12    13    13                                   h/70° C. %                                                             ______________________________________                                    

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled int he art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:
 1. A process for vulcanizing rubber which comprisesblending a sulfide derivative of an aromatic dithiocarboxylic acid ofthe formula(A--S--S--)_(z) X --S--S--A' with an organic polymeric rubberto form a mixture and heating the mixture to a temperature in the rangeof 100° C. to 250° C. wherein A and A' are, independently, amine-freearomatic thiocarbonyl groups of formula I ##STR5## wherein n, m, p, qand r are 0 or 1 and the sum of n+m+p is at least 1, R¹ and R² are,independently, C₁ -C₁₀ -alkyl radicals, alkenyl radicals with up to 10carbon atoms (C₁ -C₁₀) or aralykyl radicals with up to 15 carbon atoms,z is a natural number with 0<z<50, andx is a divalent or polyvalent(wherein z is greater than 1) linear or branched aliphatic,cycloaliphatic or araliphatic compound, or polymer having a degree ofpolymerization up to 50 and a valence of at least two,and where z=1, Xis a C₁ -C₃₀ -alkylene radical, or alkyenylene radical with up to 30carbon atoms (C₁ -C₃₀), or aralkylene radical with up to 40 carbon atomsand wherein 1<4, X is a polyvalent C₁ -C_(20z) alkane, polyvalent alkenewith up to 20z carbon atoms (C₁ -C_(20z)) or polyvalent arylalkane withup to 20z carbon atoms.
 2. A process as in claim 1, wherein the organicpolymeric rubber is styrene-butadiene copolymer rubber, polybutadienerubber, polyisoprene rubber, chloroprene rubber butyl rubber,ethylene-propylene, diene modified rubber, natural rubber, integralrubber or nitrile rubber.
 3. A process according to claim 1, whereinsaid process comprises blending1,2-bis-(4-hydroxy-3,5-ditertbutylbenzenethiocarbonyl dithio)ethane withsaid organic polymeric rubber.
 4. A process according to claim 2,wherein said process comprises blending1,2-bis-(4-hydroxy-3,5-ditertbutylbenzenethiocarbonyl dithio)ethane withsaid organic polymeric rubber.